The invention relates to a method for threading of paper or paperboard web which is of the type in which a narrow tail section separated from the paper or paperboard web (W) is conveyed between first and second holding points. The invention also relates to a threading apparatus for threading a paper or paperboard web which is adapted to guide a tail section separated from the paper or paper board web (W).
In present paper/paperboard machines the threading is conducted by cutting a narrower tail or lead-in strip from the edge of a web. This tail is conveyed through the machine all the way to the reel-up by means of threading devices (e.g. blowing plates, ropes or a suction belt). The tail can also be separated from a point closer to the middle of the web, by a so-called dual cutting. All methods, irrespective of the cutting point, share the same principle that the paper is first passed in the form of a narrower tail whereafter it is spread to form a fall-width web. The threading takes place one portion at a time, if there are free draws in the machine, wherein the tail is passed in w the form of a narrow strip to a predetermined holding point, whereafter the web is spread into the full width. From the holding point a narrow strip is conveyed to the following holding point. In this manner it is possible to conduct the threading in short sections from the press all the way to the reel-up via the drying section, wherein the distance does not grow too long and the web can still be controlled easily.
Different devices for cutting off and controlling the tail are disclosed for example in the FI patent 62695 (corresponding U.S. Pat. No. 4,501,643), U.S. Pat. No. 4,692,215, U.S. Pat. No. 4014740, FI patent 63800 (corresponding U.S. Pat. No. 4,566,944) and in the FI patent 59833 (corresponding U.S. Pat. No. 4,429,819).
Threading is most problematic along so-called free runs in which the tail/web is not supported over its entire travel path. For example in a paper web the threading from the drying section through a machine calender or a soft calender all the way to the reel-up contains relatively long free runs. In the free runs the travel of the tail from one holding point to another (e.g. calender nipxe2x80x94reel-up nip) is controlled by means of a speed difference i.e. draw difference between the points in such a manner that the latter nip in the travel direction of the tail/web has a higher speed, wherein a sufficient difference keeps the tail tight. It is a problem of this so-called speed control that it is independent of the web and its properties. Generally, the speed difference is adjusted manually by the persons operating the machine on the basis of a visual conception of the tension of the web/tail. Often the changes in the speed difference become quite substantial and they occur rapidly, wherein the risk of tail/web break is increased and production time is lost.
It is an aim of the invention to introduce a method and an apparatus by means of which the threading of paper or paperboard web can be conducted in a more reliable manner and the risk of web break is decreased.
The idea of the invention is to conduct threading in the free runs by means of tension control, wherein the runnability/tension of the strip and the subwide web can be adjusted, when necessary, within the limits of the strength of the same, thereby avoiding unnecessary breaks.
The apparatus according to the invention is characterized in what will be presented in the characterizing part of the appended claim 9.
There are known devices for measuring the web tension profile, by means of which it is possible to measure the tension profile of a full-width web during the run. This technique can also be applied in the tension control along the free run by measuring the web tension from a tail which is narrower that the full-width web. Thus, it is possible to calculate an average value from local measurement points, which average value is used as a measurement value of the tension control. The threading can also be conducted within the limits of local tension values (min and max), if necessary.